1. Field of the Invention
The present invention relates to a thin-film magnetic head for a perpendicular magnetic recording system, the thin-film magnetic head comprising a read head, and to a head gimbal assembly, a head arm assembly, and a magnetic disk drive each of which incorporates the thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
The recording systems of magnetic disk drives include a longitudinal magnetic recording system wherein signals are magnetized in the direction along the surface of the recording medium (the longitudinal direction) and a perpendicular magnetic recording system wherein signals are magnetized in the direction perpendicular to the surface of the recording medium. It is known that the perpendicular magnetic recording system is harder to be affected by thermal fluctuation of the recording medium and capable of implementing higher linear recording density, compared with the longitudinal magnetic recording system.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements.
A typical spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
A read head comprises a pair of bias field applying layers disposed on both sides of the GMR element that are opposed to each other in the direction of track width. The bias field applying layers are provided for applying a bias magnetic field to the free layer. The bias magnetic field directs the magnetization in the free layer to the direction of track width while no signal magnetic field sent from the recording medium is applied to the free layer. The magnetization in the pinned layer is fixed to the direction orthogonal to a medium facing surface of the head that faces toward the recording medium. Consequently, an angle of 90 degrees is maintained between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer while no signal field sent from the recording medium is applied to the free layer. If a signal field in the direction orthogonal to the medium facing surface is sent from the recording medium and applied to the read head, the direction of magnetization in the free layer is changed, and the angle between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer is thereby changed. The electrical resistance of the GMR element is changed by this angle. Therefore, it is possible to read data stored on the medium by detecting the change in electrical resistance of the GMR element.
The read head further comprises a pair of read shield layers disposed to sandwich the GMR element. The read shield layers are provided for preventing the GMR element from being influenced by a magnetic field from bits that are not opposed thereto.
It is known that there are types of write heads for the perpendicular magnetic recording system one of which is a single-pole head and another one of which is a shield-type head. The single-pole head comprises: a medium facing surface that faces toward a recording medium; a coil for generating a magnetic field corresponding to data to be written on the recording medium; a pole layer (main pole) having an end face located in the medium facing surface, allowing a magnetic flux corresponding to the field generated by the coil to pass therethrough, and generating a write magnetic field for writing the data on the recording medium by means of the perpendicular magnetic recording system; an auxiliary pole having an end face located in the medium facing surface and having a portion that is located away from the medium facing surface and coupled to the pole layer; and a gap layer made of a nonmagnetic material and provided between the pole layer and the auxiliary pole. In the medium facing surface the end face of the auxiliary pole is located backward of the end face of the pole layer along the direction of travel of the recording medium. The auxiliary pole has a function of returning a magnetic flux that has been generated from the end face of the pole layer and has magnetized the recording medium.
The shield-type head comprises: a medium facing surface that faces toward a recording medium; a coil for generating a magnetic field corresponding to data to be written on the recording medium; a pole layer having an end face located in the medium facing surface, allowing a magnetic flux corresponding to the field generated by the coil to pass therethrough, and generating a write magnetic field for writing the data on the recording medium by means of the perpendicular magnetic recording system; a write shield layer having an end face located in the medium facing surface and having a portion that is located away from the medium facing surface and coupled to the pole layer; and a gap layer made of a nonmagnetic material and provided between the pole layer and the write shield layer. In the medium facing surface the end face of the write shield layer is located forward of the end face of the pole layer along the direction of travel of the recording medium with a specific small space created by the thickness of the gap layer. In the shield-type head the write shield layer is capable of making the magnetic field gradient abrupt by taking in the magnetic flux generated from the pole layer. As a result, the shield-type head is capable of further improving the linear recording density. The magnetic field gradient means an amount of change of components orthogonal to the surface of the recording medium among components of the magnetic field generated from the pole layer, the amount of change being taken per unit length along the direction of travel of the recording medium. The write shield layer also has a function of returning a magnetic flux that has been generated from the end face of the pole layer and has magnetized the recording medium.
In a thin-film magnetic head for perpendicular magnetic recording, there sometimes occurs a phenomenon in which signals stored on tracks other than a track that is a target of writing or reading attenuate, resulting from the read shield layer or the write shield layer (the phenomenon will be hereinafter called track erase). It is assumed that the reason for track erase is that, in the read shield layer or the write shield layer, magnetic field components in the direction orthogonal to the medium facing surface increase locally in a neighborhood of two corner portions formed by the end face located in the medium facing surface and the respective side portions opposed to each other in the direction of track width. To secure the reliability of the thin-film magnetic head, it is required to suppress an occurrence of track erase.
Japanese Published Patent Application 2004-39148 discloses a technique for suppressing an occurrence of track erase wherein the read shield layer or the write shield layer is made to include a width changing portion that continuously decreases in width as the distance from the end face located in the medium facing surface decreases.
In the thin-film magnetic head, it is required to magnetize the bias field applying layers such that the magnetization thereof is directed to the direction of track width. This magnetizing of the bias field applying layers is performed by applying a magnetic field in the direction of track width to the thin-film magnetic head. This magnetizing is done during manufacture of the thin-film magnetic head or at the time of shipment.
The magnetizing of the bias field applying layers is performed on a plurality of occasions in some cases, such as both during manufacture of thin-film magnetic heads and at the time of shipment. Here, it has been found that, in the thin-film magnetic head comprising the read shield layer or the write shield layer including the width changing portion as mentioned above, there sometimes occurs a phenomenon in which the output of the read head varies every time the bias field applying layers are magnetized. This phenomenon does not occur if the plane geometry of the read shield layer or the write shield layer is a rectangle. To secure the reliability of the thin-film magnetic head, it is required to suppress output variations of the read head due to the magnetizing of the bias field applying layers.
In the thin-film magnetic head for perpendicular magnetic recording, there noticeably occurs a phenomenon in some cases in which signals stored on one or more tracks adjacent to a track that is the target of writing or reading attenuate (the phenomenon will be hereinafter called wide-range adjacent track erase). It is assumed that the wide-range adjacent track erase results from the instability of the magnetic state of the write shield layer. That is, in the medium facing surface, the end face of the write shield layer is located forward of the end face of the pole layer in the direction of travel of the recording medium with a specific small space created by the thickness of the gap layer. The width of the end face of the write shield layer is greater than the width of the end face of the pole layer. Therefore, it is assumed that, if the magnetic state of the write shield layer is unstable, the magnetic flux passing through the end face of the write shield layer changes, and the wide-range adjacent track erase thereby occurs. To achieve higher recording density and to secure the reliability of the thin-film magnetic head, it is also required to suppress an occurrence of the wide-range adjacent track erase.